Certain concepts lie outside our personal realms of sensory perception, but are within the realms of others. These other people may use language to share this information with us, and thus we are able to know things such as those that are listed on the Harding TOK blog. However, we will most likely never know whether those claims are true. So, my answer is that we CAN’T know these things for ourselves, we can only believe them.
A few years ago, my friend Ben Obernolte and I decided to demonstrate our debating skills and proceeded to convince a large group of naive children between ages 12 and 14 that Japan does not, in fact, exist, and that it was only a conspiracy to cover up the secret actions of China. It was quite fun.